In microwave circuits, attachment of components or circuitry to the ground plane is very critical to electrical performance of the associated device. Such attachment is typically accomplished by plated through holes or wrap around ground connections.
Plated through holes have the disadvantage of having to be placed in a particular area of the substrate. The particular location for the hole is not always known, however, and it is difficult to change the location of the plated through hole late in the process because the holes are punched in the "green state". If a plated through hole is required after the ceramic is fired, the change is effected by drilling or laser action, which can crack the ceramic substrate.
Wrap around grounds can be applied after firing the ceramic substrate and can be moved and applied at various locations around the periphery of the substrate. Traditional wrap around grounds use a conductive paste, such as silver, to make connection from the top metalization to the ground plane on the bottom surface. Once a wrap around ground is placed on the ceramic the bottom surface of the ceramic is no longer flat or planar. This lack of planarity prohibits subsequent screen printing operations and other process steps which require the substrate to be flat. In such cases, the wrap around ground must be placed on the substrate after all such process steps have been performed.
The silver paste comprising the wrap around ground thus may need to be fired at relatively low temperature (e.g., below the firing temperature of screen printed components such as resistors so as not to change their resistance value or cause oxidation). As the firing temperature of silver paste is lowered, the paste is increasingly subject to cracking and lower adhesion. The paste is particularly vulnerable to cracking and flaking at the edges of the substrate. The paste or wrap is also easily damaged by subsequent handling. The silver paste cannot be matched to the thermal coefficient of expansion of the ceramic and can crack simply due to repeated temperature cycles.
Additional wrap around ground techniques include clips. Clips, however, have the disadvantage of expense and thickness greater than 0.0254 millimeters (0.001 inches). In addition, clips are not comprised of silver (leading to galvanic action with silver metalizations), and must be preformed.
Thus, what is needed is a reinforced wrap around ground and method which provides for strong adhesion to both the circuitry to be grounded and the ground plane. The adhesion of the wrap around ground should not depend on the properties of conductive paste. The wrap around ground should not be susceptible to cracking or peeling, even from differences in the thermal coefficient of expansion between the paste and the ceramic substrate. The electrical performance of the wrap around ground should not degrade even if the fired conductive paste cracks. Finally, the wrap should not be susceptible to handling damage and should fit in the same space as a conventional wrap around ground.